A polarizing plate, used in various image display devices such as a liquid crystal display (LCD) device, a plasma display panel (PDP), and an electroluminescent (EL) device, has a structure including a polarizer formed of a polyvinyl alcohol-based film and a transparent protective film formed on at least one surface of the polarizer. At this time, a water-based adhesive is used as an adhesive for a polarizing plate that is applied to a polarizing plate to bond the polarizer and the transparent protective film, and in particular, a polyvinyl alcohol-based adhesive is used, in which a cross-linking agent is mixed with a polyvinyl alcohol-based resin aqueous solution.
When a typical polyvinyl alcohol-based resin adhesive is used, the polarizer and transparent protective film adhere to each other by means of hydrogen bonds of the polyvinyl alcohol-based resin adhesive. However, a typical polyvinyl alcohol-based resin adhesive has limitations in terms of reductions of initial adhesion, humidity resistance, and water resistance. In order to improve such limitations, an adhesive for a polarizing plate including a cross-linking agent and a polyvinyl alcohol-based resin containing an acetoacetyl group was disclosed in Japanese Patent Laid-Open Publication No. 1995-198945. Also, an adhesive composition for a polarizing plate including polyvinyl alcohol, a copolymer resin (having a degree of copolymerization ranging between 1900 and 2500) of a hydrophobic vinyl monomer such as ethyl acrylate, methyl acrylate, acrylic acid, and methacrylic acid, and a metal hardener was disclosed in Korean Patent Laid-Open Publication No. 2006-88245.
Korean Patent Laid-Open Publication No. 2006-133215 disclosed a polyvinyl alcohol-based adhesive composition including 15 to 75 percent by weight of a water-soluble acrylate-based cross-linking agent of polyvinyl alcohol. However, there are limitations in the reduction of solubility of the adhesive with respect to water, and the reduction of adhesiveness due to the addition of a large amount of a water-insoluble acrylate cross-linking agent is combined in the adhesive composition of Korean Patent Laid-Open Publication No. 2006-133215.
However, as line speed (elongation rate) has recently increased in order to improve the productivity of the polarizing plate, there are also limitations in that the humidity resistance, water resistance, and adhesion of the adhesive for a polarizing plate using the polyvinyl alcohol-based resin containing a typical acetoacetyl group may be insufficient.
Therefore, an adhesive for bonding a polarizer and a protective film having improved solubility with respect to water, excellent adhesiveness with respect to the polarizer and the protective film, humidity resistance, and water resistance is required.